Ultraviolet (UV) radiation is light which has a wavelength (λ) range of 400-100 nm and an energy per photon of 3.10 to 12.4 eV. The UV spectrum is commonly subdivided into three narrower ranges: the Ultraviolet A (UVA) range, characterized as long wave or black light, which has a wavelength range of 400-315 nm and an energy per photon of 3.10 to 3.94 eV; the Ultraviolet B (UVB) range characterized as medium wave, which has a wavelength range of 315-280 nm and an energy per photon of 3.94 to 4.43 eV; and the Ultraviolet C (UVC) range, characterized as shore wave or germicidal, which has a wavelength rage of 280-100 nm and an energy per photon of 4.43 an 12.4 eV. It is beneficial to know the amount of UV radiation due to the effect of UV radiation on living organisms. For example, UVB exposure induces the production of vitamin D in the skin of humans and a lack of exposure to UVB may lead to a lack of vitamin D. Conversely, an excess of UVB exposure can lead to direct DNA damage, sunburn, and skin cancer. Similarly, UVC can cause adverse effects that can variously be mutagenic or carcinogenic. In humans, prolonged exposure to solar UV radiation may result in acute and chronic health effects on the skin, eyes, and immune system.
To make a measurement of irradiance, it is required by definition that the response to each ray of radiation be proportional to the cosine of the angle of incidence of that ray. The ideal sensor will give a full response for rays striking the sensor perpendicularly (normal to the surface angle, 0° angle of incidence, 0° zenith angle) and conversely will give zero response for rays coming from the horizon (90° angle of incidence, 90° zenith angle). The ideal sensor will give a fifty percent (50%) response for incident rays with 60° zenith angle. For such sensors, also referred to as pyranometers, it is often useful to take measurements of light, and particularly of ultraviolet light, to determine whether concentrations of UV light are of an intensity which may be harmful. These sensors, however, must be placed in locations where incident radiation effects can introduce error into the desired measurement. In particular, errors may be introduced due to the zenith angle of light changing. Taking the vertical direction normal to the sensor as an zenith angle (Θ) equal to 0°, as the zenith angle increases and cosine Θ approaches zero, the side wall of a photosensor continues to admit light, causing a large positive cosine error relative to the measured intensity. To mitigate against this error, previous applications have used a “cutoff ring” to block light as Θ approaches 90°. The limitation of the cutoff ring is that the blocking of the light tends to be too abrupt, which can lead to irregular and erroneous measurements.
The previous applications have further tried to mitigate against such errors through use of simple light diffusers which make the intensity of light reaching a photosensor relatively uniform, but such efforts retain inaccuracies. Accordingly, there is a need in the field for an invention that can measure UV irradiance with a minimum of zenith angle cosine error stemming from structural limitations.